Functional layer including layered double hydroxide, and composite material

ABSTRACT

There is provided a functional layer including a layered double hydroxide, which is composed of: a plurality of basic hydroxide layers including Ni, Al, Ti, and OH groups; and intermediate layers composed of anions and H 2 O, each intermediate layer being interposed between two adjacent basic hydroxide layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of PCT/JP2017/022906filed Jun. 21, 2017, which claims priority to PCT/JP2017/003333 filedJan. 31, 2017, Japanese Patent Application No. 2016-125531 filed Jun.24, 2016, Japanese Patent Application No. 2016-125554 filed Jun. 24,2016, and Japanese Patent Application No. 2016-125562 filed Jun. 24,2016, the entire contents all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a functional layer including a layereddouble hydroxide, and a composite material.

2. Description of the Related Art

A layered double hydroxide (hereafter also referred to LDH) is amaterial having an exchangeable anions and H₂O as intermediate layersbetween stacked basic hydroxide layers, and is used as, for example,catalysts, adsorbents and dispersants in polymers for improving heatresistance to take its advantage.

The LDH has also been attractive as a material that conducts hydroxideions; hence, addition of the LDH to an electrolyte of an alkaline fuelcell and a catalytic layer of a zinc air battery has been studied. Inparticular, the use of a LDH as a solid electrolyte separator foralkaline secondary batteries such as nickel-zinc secondary batteries andzinc-air secondary batteries has been recently proposed, and compositematerials with a LDH containing functional layer suitable for such aseparator application are known. For example, Patent Document 1(WO2015/098610) discloses a composite material comprising a poroussubstrate and a LDH containing functional layer having no waterpermeability formed on and/or in the porous substrate. The LDHcontaining functional layer is represented by the general formula: M²⁺_(1-x)M³⁺ _(x)(OH)₂A^(n-) _(x/n).mH₂O, wherein M²⁺ is a divalent cationsuch as Mg²⁺, M³⁺ is a trivalent cation such as Al³⁺, A^(n-) is ann-valent anion such as OH⁻, CO₃ ²⁻, n is an integer of 1 or more, x is0.1 to 0.4, and m is 0 or above 0. The LDH containing functional layerdisclosed in PLT 1 is densified to such an extent that it has no waterpermeability. When the LDH is used as a separator, it can preventdeposition of dendritic zinc and penetration of carbon dioxide from anair electrode in zinc air batteries that are obstacles to practical useof alkaline zinc secondary batteries.

Unfortunately, high hydroxide ion conductivity is required forelectrolytic solutions of alkaline secondary batteries (for example,metal air batteries and nickel zinc batteries) including LDHs, and thusthe use of strong alkaline aqueous potassium hydroxide solution at pH ofabout 14 is desired. For this purpose, it is desirable for LDH to havehigh alkaline resistance such that it is barely deteriorated even insuch a strong alkaline electrolytic solution. In this regard, PatentDocument 2 (WO2016/051934) discloses a LDH containing battery thatcontains a metallic compound containing a metallic element (for example,Al) corresponding to M²⁺ and/or M³⁺ in the general formula describedabove dissolved in the electrolytic solution to suppress erosion of theLDH.

CITATION LIST Patent Documents

Patent Document 1: WO2015/098610

Patent Document 2: WO2016/051934

SUMMARY OF THE INVENTION

The present inventors have now found that it is possible to provide aLDH containing functional layer having not only high ionic conductivitybut also high alkaline resistance by employing a LDH whose basichydroxide layers are composed of predetermined elements and/or ionsincluding Ni, Al, Ti and OH groups.

Accordingly, an object of the present invention is to provide a LDHcontaining functional layer having not only high ionic conductivity butalso high alkaline resistance, and a composite material with the LDHcontaining functional layer.

According to one embodiment of the present invention, a functional layercomprising a layered double hydroxide is provided, wherein the layereddouble hydroxide is composed of: a plurality of basic hydroxide layerscomprising Ni, Al, Ti, and OH groups; and intermediate layers composedof anions and H₂O, each intermediate layer being interposed between twoadjacent basic hydroxide layers.

Another embodiment of the present invention provides a compositematerial that comprises:

-   -   a porous substrate and    -   the functional layer according to any one of claims 1 to 10        provided on the porous substrate and/or embedded in the porous        substrate.

Another embodiment of the present invention provides a battery includingthe functional layer or the composite material as a separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a LDH containingcomposite material of one embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view illustrating a LDH containingcomposite material of one another embodiment of the present invention.

FIG. 3 is a schematic view illustrating platy particles of a layereddouble hydroxide (LDH).

FIG. 4 is a schematic cross-sectional view illustrating anelectrochemical measurement system used in Examples 1 to 6.

FIG. 5A is an exploded perspective view of a hermetic container used inthe determination of density in Examples 1 to 6.

FIG. 5B is a schematic cross-sectional view of the measurement systemused in the determination of density in Examples 1 to 6.

FIG. 6A is a schematic view illustrating an example of heliumpermeability measurement system used in Examples 1 to 6.

FIG. 6B is a schematic cross-sectional view of a sample holder and itsperipheral configuration used in the measurement system shown in FIG.6A.

FIG. 7 is a graph showing an X-ray diffraction pattern of the functionallayer produced in Example 1.

FIG. 8A is an SEM image showing the surface microstructure of thefunctional layer produced in Example 1.

FIG. 8B is an SEM image showing a cross-sectional microstructure of thefunctional layer produced in Example 1.

FIG. 9 illustrates SEM images showing the surface microstructure of thefunctional layer produced in Example 1 before immersion, after immersionfor one week and after immersion for three weeks in an aqueous potassiumhydroxide solution.

FIG. 10 is a graph showing X-ray diffraction patterns of the functionallayer produced in Example 1 before immersion, after immersion for oneweek and after immersion for three weeks in an aqueous potassiumhydroxide solution.

FIG. 11 illustrates SEM images showing the cross-sectionalmicrostructure of the functional layer produced in Example 2 beforeimmersion, after immersion for three weeks and after immersion for sevenweeks in an aqueous potassium hydroxide solution.

FIG. 12A illustrates SEM images showing the cross-sectionalmicrostructure of the functional layer produced in Example 4 beforeimmersion, after immersion for three weeks and after immersion for sevenweeks in an aqueous potassium hydroxide solution.

FIG. 12B illustrates SEM images showing the surface microstructure ofthe functional layer produced in Example 4 before immersion, afterimmersion for three weeks and after immersion for seven weeks in anaqueous potassium hydroxide solution.

FIG. 13 is a BF-STEM image, an analytical pattern after FFT, and aresult of an electron analysis simulation of anatase titanium oxide fora functional layer produced in Example 4.

FIG. 14A is an SEM image showing the surface microstructure of thefunctional layer produced in Example 6 (comparative).

FIG. 14B is an SEM image showing the cross-sectional microstructure ofthe functional layer produced in Example 6 (comparative).

FIG. 15 illustrates SEM images showing the surface microstructure of thefunctional layer produced in Example 6 (comparative) before immersionand after immersion for one week in an aqueous potassium hydroxidesolution.

FIG. 16 is a graph showing X-ray diffraction patterns of the functionallayer produced in Example 6 (comparative) before immersion and afterimmersion for one week in an aqueous potassium hydroxide solution.

FIG. 17 is an SEM image showing the cross-sectional microstructure ofthe functional layer and the composite material produced in Example 7.

DETAILED DESCRIPTION OF THE INVENTION LDH Containing Functional Layerand Composite Material

The functional layer of the present invention includes a layered doublehydroxide (LDH), and the LDH is composed of: a plurality of basichydroxide layers; and intermediate layers, each of which is interposedbetween two adjacent basic hydroxide layers. The basic hydroxide layersinclude Ni, Al, Ti, and OH groups. The intermediate layers are composedof the intermediate layer composed of anions and H₂O. Although thealternately stacked structure itself of basic hydroxide layers andintermediate layers is basically the same as the commonly knownalternately stacked structure of LDH, it is possible in the presentinvention to provide a LDH containing functional layer having not onlyhigh ionic conductivity but also high alkaline resistance by employing aLDH whose basic hydroxide layers are composed of predetermined elementsand/or ions including Ni, Al, Ti and OH groups.

As described above, high alkaline resistance that barely exhibits thedeterioration even in a strong alkaline electrolytic solution is desiredfor the LDH in alkaline secondary batteries. In this respect, thefunctional layer of the present invention, a LDH whose basic hydroxidelayers are composed of predetermined elements and/or ions including Ni,Al, Ti and OH groups can exhibit high alkaline resistance. Although thereason is not clear, it is believed that Al, which has been consideredto be easily dissolved in an alkaline solution, is hard to elute intothe alkaline solution due to some interaction with Ni and Ti.Nevertheless, the functional layer of the present invention can alsoexhibit high ionic conductivity suitable for separators for alkalinesecondary batteries.

Ni in the LDH can have the form of nickel ions. Although nickel ions inthe LDH are typically believed to be Ni²⁺, they may be present in anyother valence, for example, Ni³⁺. Ti in the LDH can have the form oftitanium ions. Although titanium ions in the LDH are typically believedto be Ti⁴⁺, they may be present in any other valence, for example, Ti³⁺.Each of the incidental impurities is any element which may be inevitablymixed in a manufacturing process, and it may be mixed into the LDH from,for example, a raw material or a substrate. The intermediate layers ofthe LDH contained in the functional layer are composed of anions andH₂O. The anions are monovalent or multivalent anions, preferablymonovalent or divalent ions. Preferably, the anions in the LDH includeOH and/or CO₃ ²⁻. As described above, it is impractical or impossible tostrictly specify the LDH with a general formula since valences of Ni andTi are not necessarily confirmed. Assuming that the basic hydroxidelayers are mainly composed of Ni²⁺, Ti⁴⁺ and OH groups, the basiccomposition of the corresponding LDH can be represented by the generalformula: Ni²⁺ _(1-x)Ti⁴⁺ _(x)(OH)₂A^(n-) _(2x/n).mH₂O, wherein A^(n-) isan n-valent anion, n is an integer of 1 or more, preferably 1 or 2, x isabove 0 to below 1, preferably 0.01 to 0.5, and m is a real number of 0or more, typically a real number above 0 or 1 or more. However, itshould be understood that the general formula indicates merely the“basic composition”, and it may be replaced with other elements or ions(including elements with other valences of the same element, or elementsor ions that may be unavoidably mixed in the manufacturing process) tosuch an extent that the elements such as Ni²⁺, and Ti⁴⁺ do not impairthe basic properties of LDH.

In the functional layer, the atomic ratio Ti/(Ni+Ti+Al) determined byenergy dispersive X-ray analysis (EDS), is preferably 0.10 to 0.90, morepreferably 0.20 to 0.80, further preferably 0.25 to 0.70, particularlypreferably 0.30 to 0.61. Within these ranges, both the alkalineresistance and the ion conductivity can be improved. Accordingly, thefunctional layer may contain not only LDH but also Ti in an amount largeenough to produce titania as a by-product. That is, the functional layermay further include titania. Inclusion of titania leads to highhydrophilicity, which is expected to improve wettability with anelectrolytic solution (namely to improve conductivity).

The LDH contained in the functional layer preferably undergoes nochanges in the surface microstructure and crystalline structure whenimmersed in a 5 mol/L aqueous potassium hydroxide solution containingzinc oxide in a concentration of 0.4 mol/L at 70° C. for three weeks or504 hours because such an LDH has high alkaline resistance. The presenceof a change in the surface microstructure can be preferably determinedby SEM (Scanning Electron Microscopy), and the presence of a change inthe crystalline structure can be preferably determined by crystallinestructural analysis (for example, a shift in (003) peak) by XRD (X-raydiffractometry).

The functional layer (in particular, the LDH contained in the functionallayer) has preferably hydroxide ion conductivity. The functional layerhas preferably an ionic conductivity of at least 0.1 mS/cm, morepreferably at least 0.5 mS/cm, and most preferably at least 1.0 mS/cm.Higher ionic conductivity is preferred. The upper limit thereof is forexample, 10 mS/cm, which should not be construed as limiting. Such highionic conductivity is particularly suitable for battery application. Forexample, it is preferred to lower the resistance by thinning in order toput the LDH into practical use, and providing the LDH containingfunctional layers with desirably low resistance according to theembodiment is particularly advantageous in the application of LDH as asolid electrolyte separator for alkaline secondary batteries such aszinc air batteries or nickel zinc batteries.

Preferably, the functional layer is disposed on the porous substrateand/or embedded into the porous substrate. That is, a preferredembodiment of the present invention provides a composite materialcomprising a porous substrate and a functional layer disposed on theporous substrate and/or embedded into the porous substrate. For example,as in the composite material 10 shown in FIG. 1, a part of thefunctional layer 14 may be embedded in the porous substrate 12 and theremaining part may be disposed on the porous substrate 12. In this case,the portion of the functional layer 14 on the porous substrate 12 is amembrane portion made of a LDH membrane, and the portion of thefunctional layer 14 embedded into the porous substrate 12 is a compositeportion composed of the porous substrate and the LDH. The compositeportion is typically in the form in which the inside of the pores of theporous substrate 12 is filled with the LDH. Also, in the case where theentire functional layer 14′ is embedded in the porous substrate 12 asthe composite material 10′ shown in FIG. 2, the functional layer 14′ ismainly composed of the porous substrate 12 and the LDH. The compositematerial 10′ and the functional layer 14′ shown in FIG. 2 can be formedby removing the membrane portion (LDH membrane) of the functional layer14 from the composite material 10 shown in FIG. 1 by a known method suchas polishing or cutting. In FIGS. 1 and 2, the functional layers 14, 14′are embedded only in a part of the vicinity of the surface of the poroussubstrates 12, 12′, but the functional layers may be embedded in anypart of the porous substrate and over the entire part or the entirethickness of the porous substrate.

The porous substrate in the composite material of the present inventioncan preferably form the LDH containing functional layer thereon and/ortherein. The substrate may be composed of any material and have anyporous structure. Although it is typical to form the LDH containingfunctional layer on and/or in the porous substrate, the LDH containingfunctional layer may be formed on a non-porous substrate and then thenon-porous substrate may be modified into a porous form by any knownmethod. In any case, the porous substrate has preferably a porousstructure having good water permeability and it can deliver electrolyticsolution to the functional layer when incorporated into a battery as aseparator for the battery.

The porous substrate is composed of, preferably, at least one selectedfrom the group consisting of ceramic materials, metallic materials, andpolymeric materials, more preferably, at least one selected from thegroup consisting of ceramic materials and polymeric materials. Morepreferably, the porous substrate is composed of ceramic materials. Inthis case, preferred examples of the ceramic material include alumina,zirconia, titania, magnesia, spinel, calcia, cordierite, zeolite,mullite, ferrite, zinc oxide, silicon carbide, and any combinationthereof. More preferred examples include alumina, zirconia, titania, andany combination thereof. Particularly preferred examples includealumina, zirconia (for example, yttria-stabilized zirconia (YSZ)), andcombination thereof. Using these porous ceramics, a LDH containingfunctional layer with high density can be readily formed. Preferredexamples of the metallic material include aluminum, zinc, and nickel.Preferred examples of the polymeric material include polystyrene,polyethersulfone, polypropylene, epoxy resin, poly(phenylene sulfide),hydrophilized fluororesin (such as tetrafluoro resin: PTFE), cellulose,nylon, polyethylene and any combination thereof. All these preferredmaterials have high resistance to the alkaline electrolytic solution ofthe battery.

Further preferably, the porous substrate is composed of the polymericmaterial. The polymeric porous substrate has the following advantageousproperties; (1) high flexibility (hard to crack even if thinned), (2)high porosity, (3) high conductivity (thin thickness with highporosity), and (4) good manufacturability and handling ability. Furtherpreferred polymeric materials are polyolefins such as, for example,polypropylene, polyethylene, and most preferably polypropylene from theviewpoint of high resistance to hot water, high acid resistance and highalkaline resistance, as well as low cost. When the porous substrate iscomposed of the polymeric material, it is more preferred that thefunctional layer is embedded into the entire porous substrate over thethickness (for example, most or substantially all of the pores insidethe porous substrate are filled with the LDH). The preferred thicknessof the polymeric porous substrate in this case is 5 to 200 μm, morepreferably 5 to 100 μm, most preferably 5 to 30 μm. Usable polymerporous substrates are microporous membranes commercially available asseparators for lithium batteries.

The porous substrate has preferably a mean pore diameter of at most 100μm, more preferably at most 50 μm, for example, typically 0.001 to 1.5μm, more typically 0.001 to 1.25 μm, further more typically 0.001 to 1.0μm, particularly typically 0.001 to 0.75 μm, most typically 0.001 to 0.5μm. Within these ranges, a dense LDH containing functional layer havingno water permeability can be formed while keeping desirable waterpermeability and strength as a support for the porous substrate. In thepresent invention, the mean pore size can be determined by measuring thelargest dimension of each pore based on the electron microscopic imageof the surface of the porous substrate. The electron microscopic imageis measured at 20,000-fold magnification or more. All the measured poresizes are listed in order of size to calculate the average, from whichthe subsequent 15 larger sizes and the subsequent 15 smaller sizes,i.e., 30 diameters in total, are selected in one field of view. Theselected sizes of two fields of view are then averaged to yield theaverage pore size. In the measurement, a dimension measuring function insoftware of SEM or image analyzing software (for example, Photoshopmanufactured by Adobe) can be used.

The porous substrate has a porosity of preferably 10 to 60%, morepreferably 15 to 55%, most preferably 20 to 50%. Within these ranges,the resulting dense LDH containing functional layer has no waterpermeability while the porous substrate keeps desirable waterpermeability and required strength as a support. The porosity of theporous substrate can be preferably measured by Archimedes' method. Inthe case where the porous substrate is composed of the polymericmaterial and the functional layer is embedded over the region of theporous substrate in the thickness direction, the porosity of the poroussubstrate is preferably 30 to 60%, more preferably 40 to 60%.

The functional layer preferably has no air permeability. That is, it ispreferred that the functional layer be densified with the LDH to such anextent that it has no air permeability. In the present specification,the phrase “having no air permeability” has the following meaning: Inthe case of evaluation of air permeability by the “density determinationtest” adopted in the examples described later or a similar method, nobubbling of helium gas is observed at one side of the measured object,i.e., the functional layer and/or the porous substrate even if heliumgas is brought into contact with the other side in water at adifferential pressure of 0.5 atm across the thickness. By thisdensification, the functional layer or the composite material as a wholeselectively allows only the hydroxide ion due to its hydroxide ionconductivity to pass through, and can function as separators forbatteries. In the case of the application of LDH as solid electrolyteseparators for batteries, although the bulk LDH dense body has highresistance, the LDH containing functional layer in a preferredembodiment of the present invention can be thinned to reduce theresistance because the porous substrate has high strength. In addition,the porous substrate can have high water permeability and airpermeability; hence, the electrolytic solution can reach the LDHcontaining functional layer when used as solid electrolyte separators ofbatteries. In summary, the LDH containing functional layer and thecomposite material of the present invention are very useful materialsfor solid electrolyte separators applicable to various batteries, suchas metal air batteries (for example, zinc air batteries) and variousother zinc secondary batteries (for example, nickel zinc batteries).

In the functional layer or the composite material including thefunctional layer, a helium permeability per unit area is preferably 10cm/min·atm or less, more preferably 5.0 cm/min·atm or less, mostpreferably 1.0 cm/min·atm or less. The functional layer having such arange of helium permeability has extremely high density. When thefunctional layer having a helium permeability of 10 cm/min·atm or lessis applied as a separator in an alkaline secondary battery, passage ofsubstances other than hydroxide ions can be effectively prevented. Forexample, zinc secondary batteries can significantly effectively suppresspenetration of Zn in the electrolytic solution. Since penetration of Znis remarkably suppressed in this way, it can be believed in principlethat deposition of dendritic zinc can be effectively suppressed in zincsecondary batteries. The helium permeability is measured throughsupplying helium gas to one surface of the functional layer to allowhelium gas to pass through the functional layer and calculating thehelium permeability to evaluate density of the functional layer. Thehelium permeability is calculated from the expression of F/(P×S) where Fis the volume of permeated helium gas per unit time, P is thedifferential pressure applied to the functional layer when helium gaspermeates through, and S is the area of the membrane through whichhelium gas permeates. Evaluation of the permeability of helium gas inthis manner can extremely precisely determine the density. As a result,a high degree of density that does not permeate as much as possible (orpermeate only a trace amount) substances other than hydroxide ions (inparticular, zinc that causes deposition of dendritic zinc) can beeffectively evaluated. Helium gas is suitable for this evaluationbecause the helium gas has the smallest constitutional unit amongvarious atoms or molecules which can constitute the gas and itsreactivity is extremely low. That is, helium does not form a molecule,and helium gas is present in the atomic form. In this respect, sincehydrogen gas is present in the molecular form (H₂), atomic helium issmaller than molecular H₂ in a gaseous state. Basically, H₂ gas iscombustible and dangerous. By using the helium gas permeability definedby the above expression as an index, the density can be objectively andreadily evaluated regardless of differences in sample size andmeasurement condition. Thus, whether the functional layer hassufficiently high density suitable for separators of zinc secondarybatteries can be evaluated readily, safely and effectively. The heliumpermeability can be preferably measured in accordance with the procedureshown in Evaluation 5 in Examples described later.

The LDH includes agglomerates of platy particles (that is, LDH platyparticles), and surfaces of platy particles are preferably orientedperpendicular or oblique to the surface of the functional layer (thesurface when macroscopically observed to such an extent that the fineirregularities of the functional layer can be ignored). In the case thatthe functional layer is provided on the porous substrate, the functionallayer has a membrane portion provided above the porous substrate. Inthis case, the LDH constituting the membrane portion includesagglomerates of platy particles (that is, LDH platy particles), andsurfaces of platy particles are preferably oriented perpendicular oroblique to the surface of the porous substrate (the surface of theporous substrate when macroscopically observed to such an extent thatthe fine irregularities due to the porous structure can be ignored). Inthe case that the functional layer has a composite portion to beembedded in the porous substrate, the LDH platy particles may also bepresent in the pores of the porous substrate. Also, the membrane portionmay further contain ceramic particles such as alumina particles as afiller to increase the adhesive strength between the LDH in the membraneportion and the substrate.

It is known that LDH crystals have the form of platy particles includinga layered structure as shown in FIG. 3, and the perpendicular or obliqueorientation is very advantageous to the LDH containing functional layers(for example LDH dense membranes). Because the oriented LDH containingfunctional layer (for example, oriented LDH dense membrane) hasanisotropic conductivity. In detail, the ionic conductivity of hydroxidein the direction of the aligned LDH platy particles (that is, thedirection parallel to the layer of LDH) is extremely higher than theconductivity in the direction orthogonal to the above aligned direction.In fact, it is already known that the conductivity (S/cm) in thealigning direction in the oriented bulk of the LDH is one order ofmagnitude higher than the conductivity (S/cm) in the directionorthogonal to the aligning direction. That is, the perpendicular oroblique orientation in the LDH containing functional layer of thepresent invention causes maximal or significant anisotropy of theconductivity of the oriented LDH in the layer thickness direction (thatis, in the direction perpendicular to the surface of the functionallayer or the porous substrate). As a result, the conductivity in thelayer thickness direction can be maximally or significantly increased.In addition, the LDH containing functional layer is in a layer form;hence, it has a lower resistance than the LDH in a bulk form. The LDHcontaining functional layer having such an orientation can facilitatethe conduction of hydroxide ions in the layer thickness direction.Moreover, the LDH containing functional layer is densified; hence, it isextremely useful in a functional membrane such as battery separators(for example, a hydroxide ion conductive separator for zinc airbatteries) that requires high conductivity and density in the layerthickness direction.

The functional layer has preferably a thickness of 100 μm or less, morepreferably 75 μm or less, further preferably 50 μm or less, particularlypreferably 25 μm or less, most preferably 5 μm or less. Such thinningcan reduce the resistance of the functional layer. In the case where thefunctional layer is formed as the LDH membrane on the porous substrate,the thickness of the functional layer corresponds to the thickness ofthe membrane portion composed of the LDH membrane. In the case where thefunctional layer is formed to be embedded into the porous substrate, thethickness of the functional layer corresponds to the thickness of thecomposite potion composed of the porous substrate and the LDH. In thecase where the functional layer is formed on and in the poroussubstrate, the thickness of the functional layer corresponds to thetotal thickness of the membrane portion (the LDH membrane) and thecomposite portion (the porous substrate and the LDH). In any case, theabove thickness leads to a low resistance suitable for practical use in,for example, battery application. Although the lower limit of thethickness of the LDH membrane is not limited because it depends on theapplication, the thickness is preferably 1 μm or more, more preferably 2μm or more in order to assure a certain degree of rigidity suitable fora functional membrane such as a separator.

The LDH containing functional layer and the composite material can beproduced by any method. They can be produced by appropriately modifyingconditions of a known method for producing LDH containing functionallayers and composite materials (see, for example, PLT 1 and 2). Forexample, the LDH containing functional layer and the composite materialcan be produced by (1) providing a porous substrate, (2) applying amixed sol of alumina and titania onto the porous substrate and thenheating the sol to form an alumina/titania layer, (3) immersing theporous substrate into an aqueous raw material solution containing nickelions (Ni²⁺) and urea, and (4) hydrothermally treating the poroussubstrate in the aqueous raw material solution to form the LDHcontaining functional layer on the porous substrate and/or in a poroussubstrate. In particular, in Step (2), forming the alumina/titania layeron the porous substrate can not only produce a raw material for the LDH,but also serve as a seed of LDH crystalline growth and uniformly formthe LDH containing functional layer that is highly densified on thesurface of the porous substrate. In addition, in Step (3), the presenceof urea raises the pH value through generation of ammonia in thesolution through the hydrolysis of urea, and gives the LDH by formationof hydroxide with coexisting metal ions. Also, generation of carbondioxide in hydrolysis gives the LDH of a carbonate anion type.

In particular, a composite material in which the porous substrate iscomposed of a polymeric material and the functional layer is embeddedover the porous substrate in the thickness direction is produced byapplying the mixed sol of alumina and titania to the substrate in Step(2) in such that the mixed sol permeates into all or most area of theinterior pores of the substrate. By this manner, most or substantiallyall pores inside the porous substrate can be embedded with the LDH.Examples of preferred application include dip coating and filtrationcoating. Particularly preferred is dip coating. The amount of thedeposited mixed sol can be varied by adjusting the number of times ofcoating such as dip coating. The substrate coated with the mixed sol by,for example, dip coating may be dried and then subjected to Steps (3)and (4).

EXAMPLES

The present invention will be described in more detail by the followingexamples. The functional layers and the composite materials produced inthe following examples were evaluated as follows:

Evaluation 1: Identification of Functional Layer

The crystalline phase of the functional layer was measured with an X-raydiffractometer (RINT TTR III manufactured by Rigaku Corporation) at avoltage of 50 kV, a current of 300 mA, and a measuring range of 10° to70° to give an XRD profile. The resultant XRD profile was identifiedwith the diffraction peaks of LDH (hydrotalcite compound) described inJCPDS card NO. 35-0964.

Evaluation 2: Observation of Microstructure

The surface microstructure of the functional layer was observed at anaccelerating voltage of 10 to 20 kV with a scanning electron microscope(SEM, JSM-6610LV, manufactured by JEOL Ltd.). After preparation of across-sectional polished surface of the functional layer (a membraneportion composed of a LDH membrane and a composite portion composed ofthe LDH and the substrate) with an ionic milling system (IM4000,manufactured by Hitachi High-Technologies Corporation), themicrostructure of the cross-sectional polished surface was observed withthe SEM under the same conditions.

Evaluation 3: Elemental Analysis (EDS) I

The functional layer (the membrane portion composed of the LDH membraneand the composite portion composed of the LDH and the substrate) waspolished across the thickness for observation with a cross-sectionalpolisher (CP). A field of cross-sectional image of the functional layer(the membrane portion composed of the LDH membrane and the compositeportion composed of the LDH and the substrate) was observed with a10,000-fold magnification with FE-SEM (ULTRA 55, manufactured by CarlZeiss). The LDH membrane on the substrate surface and the LDH portion(by point analysis) inside the substrate in this cross-sectional imagewas subjected to elemental analysis at an accelerating voltage of 15 kVwith an EDS analyzer (NORAN System SIX, manufactured by Thermo FisherScientific Inc.).

Evaluation 4: Elemental Analysis (EDS) II

Compositional analysis was performed on the surface of the functionallayer with an EDS analyzer (a brand name of the analyzer: X-act,manufactured by Oxford Instruments plc) to calculate an atomic ratioTi/(Ni+Ti+Al). This analysis comprises 1) taking an image at anacceleration voltage of 20 kV with 5,000-fold magnification, 2) analysisin a point analytic mode at three points spaced with about 5 μm eachother, 3) repeating Steps 1) and 2) further two times, and 4)calculation of the average value of nine points in total.

Evaluation 5: Evaluation of Alkaline Resistance

Zinc oxide was dissolved in 6 mol/L of aqueous potassium hydroxidesolution to yield 5 mol/L of aqueous potassium hydroxide solution thatcontained 0.4 mol/L of zinc oxide. In the next stage, 15 mL of theresultant aqueous potassium hydroxide solution was placed in a hermeticcontainer made of Teflon™. A composite material having dimensions of 1cm×0.6 cm was placed on the bottom of the hermetic container such thatthe functional layer faced upward, and the lid was closed. The compositematerial was held at 70° C. (Examples 1 to 5) or 30° C. (Example 6) forone week (168 hours), three weeks (504 hours) or 7 weeks (1176 hours)and then removed from the hermetic container. The composite material wasdried overnight at room temperature. The microstructure of the resultantsample was observed with SEM and the crystalline structure was analyzedwith XRD. In the analysis of crystalline structure by XRD, if a shift ofthe peak (26) beyond 0.25° with respect to the (003) peak of LDH occursafter immersion in the aqueous potassium hydroxide solution, thecrystalline structure was determined to be significantly changed.

Evaluation 6: Measurement of Ionic Conductivity

The conductivity of the functional layer in the electrolytic solutionwas measured with an electrochemical measurement system shown in FIG. 4.A composite material sample S (a porous substrate with an LDH membrane)was sandwiched between two silicone gaskets 40 having a thickness of 1mm and assembled into a PTFE flange-type cell 42 having an innerdiameter of 6 mm. Electrodes 46 made of #100 nickel wire mesh wereassembled into a cylinder having a diameter of 6 mm in the cell 42, andthe distance between the electrodes was 2.2 mm. The cell 42 was filledwith an aqueous electrolytic solution 44 containing 6M potassiumhydroxide. Using electrochemical measurement system(potentio-galvanostat frequency responsive analyzers 1287A and 1255Bmanufactured by Solartron), the sample was observed under the conditionsof a frequency range of 1 MHz to 0.1 Hz and an applied voltage of 10 mV,and the resistance of the composite material sample S (the poroussubstrate with LDH membrane) was determined from the intercept across areal number axis. The resistance of the porous substrate without the LDHmembrane was also measured in the same manner. The resistance of the LDHmembrane was determined from the difference in resistance between thecomposite material sample S (the porous substrate with the LDH membrane)and the substrate. The conductivity was determined with the resistance,the thickness, and the area of the LDH membrane.

Evaluation 7: Determination of Density

The density was determined to confirm that the functional layer haddensity having no air permeability. As shown in FIGS. 5A and 5B, an openacrylic container 130 and an alumina jig 132 with a shape and dimensionscapable of working as a lid of the acrylic container 130 were provided.The acrylic container 130 was provided with a gas supply port 130 a. Thealumina jig 132 had an opening 132 a having a diameter of 5 mm and acavity 132 b surrounding the opening 132 a for placing the sample. Anepoxy adhesive 134 was applied onto the cavity 132 b of the alumina jig132. The composite material sample 136 was placed into the cavity 132 band the function layer 136 b was bonded to the alumina jig 132 in anair-tight and liquid-tight manner. The alumina jig 132 with thecomposite material sample 136 was then bonded to the upper end of theacrylic container 130 in an air-tight and liquid-tight manner with asilicone adhesive 138 to completely seal the open portion of the acryliccontainer 130. A hermetic container 140 was thereby completed for themeasurement. The hermetic container 140 for the measurement was placedin a water vessel 142 and the gas supply port 130 a of the acryliccontainer 130 was connected to a pressure gauge 144 and a flow meter 146so that helium gas was supplied into the acrylic container 130. Water143 was poured in the water vessel 142 to completely submerge thehermetic container 140 for the measurement. At this time, theair-tightness and liquid-tightness were sufficiently kept in theinterior of the hermetic container 140 for the measurement, and thefunctional layer 136 b of the composite material sample 136 was exposedto the internal space of the hermetic container 140 for the measurementwhile the porous substrate 136 a of the composite material sample 136was in contact with water in the water vessel 142. In this state, heliumgas was introduced into the acrylic container 130 of the hermeticcontainer 140 for the measurement through the gas supply port 130 a. Thepressure gauge 144 and the flow meter 146 were controlled such that thedifferential pressure between the inside and outside of the functionallayer 136 a reached 0.5 atm (that is, the pressure of the helium gas is0.5 atm higher than the water pressure applied to the porous substrate136 a). Bubbling of helium gas in water from the composite materialsample 136 was observed. When bubbling of helium gas was not observed,the functional layer 136 b was determined to have high density with noair permeability.

Evaluation 8: Helium Permeability

A helium permeation test was conducted to evaluate the density of thefunctional layer from the viewpoint of helium permeability. The heliumpermeability measurement system 310 shown in FIGS. 6A and 6B wasconstructed. The helium permeability measurement system 310 wasconfigured to supply helium gas from a gas cylinder filled with heliumgas to a sample holder 316 through the pressure gauge 312 and a flowmeter 314 (digital flow meter), and to discharge the gas by permeatingfrom one side to the other side of the functional layer 318 held by thesample holder 316.

The sample holder 316 had a structure including a gas supply port 316 a,a sealed space 316 b and a gas discharge port 316 c, and was assembledas follows: An adhesive 322 was applied along the outer periphery of thefunctional layer 318 and bonded to a jig 324 (made of ABS resin) havinga central opening. Gaskets or sealing members 326 a, 326 b made of butylrubber were disposed at the upper end and the lower end, respectively,of the jig 324, and then the outer sides of the members 326 a, 326 bwere held with supporting members 328 a, 328 b (made of PTFE) eachincluding a flange having an opening. Thus, the sealed space 316 b waspartitioned by the functional layer 318, the jig 324, the sealing member326 a, and the supporting member 328 a. The functional layer 318 was inthe form of a composite material formed on the porous substrate 320, andwas disposed such that the functional layer 318 faced the gas supplyport 316 a. The supporting members 328 a and 328 b were tightly fastenedto each other with fastening means 330 with screws not to cause leakageof helium gas from portions other than the gas discharge port 316 c. Agas supply pipe 34 was connected to the gas supply port 316 a of thesample holder 316 assembled as above through a joint 332.

Helium gas was then supplied to the helium permeability measurementsystem 310 via the gas supply pipe 334, and the gas was permeatedthrough the functional layer 318 held in the sample holder 316. A gassupply pressure and a flow rate were then monitored with a pressuregauge 312 and a flow meter 314. After permeation of helium gas for oneto thirty minutes, the helium permeability was calculated. The heliumpermeability was calculated from the expression of F/(P×S) where F(cm³/min) was the volume of permeated helium gas per unit time, P (atm)was the differential pressure applied to the functional layer whenhelium gas permeated through, and S (cm²) was the area of the membranethrough which helium gas permeates. The permeation rate F (cm³/min) ofhelium gas was read directly from the flow meter 314. The gauge pressureread from the pressure gauge 312 was used for the differential pressureP. Helium gas was supplied such that the differential pressure P waswithin the range of 0.05 to 0.90 atm.

Evaluation 9: Identification of Titania

A BF-STEM image of the functional layer was taken with a scanningtransmission electron microscope (STEM) (a brand name: JEM-ARM200F,manufactured by JEOL). The BF-STEM image was subjected to Fast FourierTransform (FFT) analysis to give an analytical pattern after FFT. Theresultant analytical pattern was compared with the result of theelectron analysis simulation of the anatase titanium oxide and it wasthen confirmed whether the lattice constant read from the analyticalpattern after FFT roughly corresponds to the anatase titanium oxide.

Example 1 to 5

A functional layer including Ni/Al/Ti-containing LDH and a compositematerial were prepared and evaluated by a following procedure.

(1) Preparation of Porous Substrate

One hundred parts by weight of zirconia powder (TZ-8YS manufactured byTosoh Corporation), 70 parts by weight of a dispersing medium(xylene:butanol=1:1), 11.1 parts by weight of a binder (polyvinylbutyral: BM-2 manufactured by Sekisui Chemical Co., Ltd.), 5.5 parts byweight of a plasticizer (DOP manufactured by Kurogane Kasei Co., Ltd.),and 2.9 parts by weight of a dispersant (Rheodol SP-030 manufactured byKao Corporation) were mixed, and the mixture was stirred to be deformedunder reduced pressure to yield a slurry. The slurry was shaped into asheet on a PET film with a tape shaping machine to yield a green sheethaving the membrane thickness of 220 μm after drying. The green sheetwas cut into 2.0 cm×2.0 cm×0.022 cm and fired at 1100° C. for two hoursto yield a porous substrate made of zirconia.

The porosity of the porous substrate was measured to be 40% byArchimedes' method.

The observed mean pore size of the porous substrate was 0.2 μm. In thepresent invention, the mean pore size was determined by measuring thelongest dimension of each pore based on the scanning electronmicroscopic (SEM) image of the surface of the porous substrate. The SEMimage was observed at 20,000-fold magnification. All the measured poresizes are listed in order of size to calculate the average, from whichthe subsequent 15 larger sizes and the subsequent 15 smaller sizes,i.e., 30 diameters in total, are selected in one field of view. Theselected sizes of two fields of view are then averaged to yield theaverage pore size. In the measurement, a dimension measuring function insoftware of SEM was used.

(2) Coating of Alumina/Titania Sol on Porous Substrate

An amorphous alumina solution (AI-ML15 manufactured by Taki ChemicalCo., Ltd.) and a titanium oxide sol solution (M6 manufactured by TakiChemical Co., Ltd.) (weight ratio of 1:1) were mixed at Ti/AI molarratios shown in Table 1 to prepare a mixed sol. The zirconia poroussubstrate prepared in Procedure (1) was coated with 0.2 mL of the mixedsol by spin coating. In the spin coating, the mixed sol was dropwiseadded to the substrate spinning at a rotation rate of 8,000 rpm, thenthe spin was stopped after five seconds. The substrate was placed on ahot plate heated to 100° C. and dried for one minute. The substrate washeated at 300° C. in an electric furnace. The thickness of the layerformed by this procedure was about 1 μm.

(3) Preparation of Aqueous Raw Material Solution

Nickel nitrate hexahydrate (Ni (NO₃)₂.6H₂O, manufactured by KantoChemical CO., Inc.), and urea ((NH₂)₂CO, manufactured by Sigma-AldrichCorporation) were provided as raw materials. Nickel nitrate hexahydratewas weighed to be 0.015 mol/L and placed in a beaker, and ion-exchangedwater was added thereto into a total amount of 75 mL. After stirring thesolution, the urea weighed at a urea/NO₃ ⁻ molar ratio of 16 was added,and further stirred to give an aqueous raw material solution.

(4) Formation of Membrane by Hydrothermal Treatment

The aqueous raw material solution prepared in Procedure (3) and thesubstrate prepared in Procedure (2) were placed in a Teflon™ hermeticcontainer (autoclave, the internal volume: 100 mL, and covered withstainless steel jacket). The substrate was horizontally fixed away fromthe bottom of Teflon™ hermetic container such that the solution was incontact with the two surfaces of the substrate. A LDH was then formed onthe surface and the interior of the substrate by a hydrothermaltreatment at a temperature of 150° C. for 72 hours (Example 1) or 120°C. for 24 hours (Example 2 to 5). After a predetermined period, thesubstrate was removed from the hermetic container, washed withion-exchanged water, and dried at 70° C. for ten hours to yield a LDHcontaining functional layer partly embedded in the porous substrate. Thethickness of the functional layer was about 5 μm (including thethickness of the portion embedded in the porous substrate).

(5) Results of Evaluation

Evaluations 1 to 8 were performed on the resultant functional layer orcomposite material. Evaluation 9 was performed on Example 4 only. Theresults were as follows.

-   -   Evaluation 1: The XRD profile identifies the functional layer        prepared in Example 1 to 5 as a LDH (hydrotalcite compound).        FIG. 7 illustrates an XRD profile taken in Example 1. FIG. 7        also shows peaks derived from zirconia in the porous substrate.    -   Evaluation 2: FIGS. 8A and 8B shows SEM images of the surface        microstructure and the cross-sectional microstructure of the        functional layer, respectively. FIG. 8B demonstrates that the        functional layer includes a membrane portion composed of a LDH        membrane and a composite portion composed of the LDH and the        porous substrate located below the membrane portion.        Furthermore, the LDH of the membrane portion is composed of        agglomerates of platy particles, and these platy particles were        oriented perpendicular or oblique to the surface of the porous        substrate (the surface of the porous substrate when        macroscopically observed to such an extent that the fine        irregularities due to the porous structure can be ignored). In        contrast, the pores of the porous substrate are filled with the        LDH in the composite portion to form a dense layer. The surface        microstructure and cross-sectional microstructure of the        functional layers in Examples 2 to 5 were generally the same as        those in Example 1.    -   Evaluation 3: The results of EDS elemental analysis indicate C,        Al, Ti and Ni that are constituent elements of the LDH are        detected in the LDH contained in the functional layer or in both        the LDH membrane on the substrate surface and the LDH portion in        the substrate. Al, Ti and Ni are constituent elements of the        basic hydroxide layer while C corresponds to CO₃ ²⁻ that is an        anion constituting the intermediate layer of LDH.    -   Evaluation 4: Table 1 shows the atomic ratio of Ti/(Ni+Ti+Al) of        the surface of each functional layer calculated by EDS elemental        analysis.    -   Evaluation 5: Table 1 shows the results of the SEM observation        results in Examples 1 to 5. FIG. 9 illustrates SEM images        showing the surface microstructure of the functional layer of        Example 1 before immersion, after immersion for one week and        after immersion for three weeks in an aqueous potassium        hydroxide solution. In addition, FIGS. 11 and 12A illustrate SEM        images showing the cross-sectional microstructure of the        functional layer of Example 2 and 4 before immersion, after        immersion for three weeks and after immersion for seven weeks in        an aqueous potassium hydroxide solution, while FIG. 12B        illustrates an SEM image showing the surface microstructure of        the functional layer of Example 4. As shown in Table 1, FIGS. 9,        11, 12A and 12B, no change in the microstructure of the        functional layers of Examples 1 to 5 is observed even after        immersion in the aqueous potassium hydroxide solution at 70° C.        for three weeks. In particular, as shown in Table 1, FIGS. 12A        and 12B, the samples of Examples 3 to 5 with high Ti/(Ni+Ti+Al)        ratios resulted in no change in the microstructure observed even        after immersion in the aqueous potassium hydroxide solution at        70° C. for seven weeks. The immersion in an aqueous potassium        hydroxide solution at a high temperature of 70° C. can be a        considerably severe alkaline resistance test, as compared to the        immersion at a low temperature of 30° C. Accordingly, even        resistance to immersion for three weeks can be evaluated as        sufficient alkaline resistance; however, resistance to immersion        for seven weeks can be evaluated as particularly superior        alkaline resistance. Although the microstructure of the LDH        membrane of Example 2 shown in FIG. 11 changed significantly        after immersion in the aqueous potassium hydroxide solution for        seven weeks, the density of the composite material as a whole        are considered to be still ensured, because the pores of the        porous substrate as the base of the LDH membrane are also filled        with LDH.

Table 1 shows the results of SEM observation in Examples 1 to 5. FIG. 10illustrates the X-ray diffraction results of the functional layer beforeimmersion, after immersion for one week and after immersion for threeweeks in the aqueous potassium hydroxide solution. As shown in Table 1and FIG. 10, no significant change in the crystalline structure isobserved even after immersion in the aqueous potassium hydroxidesolution at 70° C. for three weeks in all Examples 1 to 5. Inparticular, as shown in Table 1, no significant change in thecrystalline structure is observed even after immersion in the aqueouspotassium hydroxide solution at 70° C. for seven weeks in the samples ofExamples 2 to 5, with high Ti/(Ni+Ti+Al) ratios. In fact, the (003) peakof LDH contained in the functional layer of Example 1 is found at2θ=11.36 in all the functional layers before immersion, after immersionfor one week and after immersion for three weeks in the aqueouspotassium hydroxide solution.

-   -   Evaluation 6: Ionic conductivities of the functional layers of        Examples 1 to 5 were 2.0 to 2.5 mS/cm equivalent to that of        Example 6 which is a comparative example described later.    -   Evaluation 7: The functional layers and the composite materials        of Examples 1 to 5 were confirmed to have high density with no        air permeability.    -   Evaluation 8: Helium permeabilities through the functional        layers and the composite materials of Examples 1 to 5 were 0.0        cm/min·atm.    -   Evaluation 9: FIG. 13 illustrates the BF-STEM image and the        analytic pattern after FFT of the functional layer in Example 4.        Since lattice constants read from the analytical patterns after        FFT in the functional layer generally correspond to the        simulated electron analysis pattern of anatase titanium oxide,        the functional layer was confirmed to contain anatase titania.

Example 6 (Comparative)

A functional layer including Mg/Al-containing LDH and a compositematerial were prepared and evaluated by a following procedure.

(1) Preparation of Porous Substrate

One hundred parts by weight of alumina powder (AES-12 manufactured bySumitomo Chemical Co., Ltd.), 70 parts by weight of a dispersing medium(xylene:butanol=1:1), 11.1 parts by weight of a binder (polyvinylbutyral: BM-2 manufactured by Sekisui Chemical Co., Ltd.), 5.5 parts byweight of a plasticizer (DOP manufactured by Kurogane Kasei Co., Ltd.),and 2.9 parts by weight of a dispersant (Rheodol SP-O30 manufactured byKao Corporation) were mixed, and the mixture was stirred to be deformedunder vacuum to yield a slurry. The slurry was shaped into a sheet on aPET film with a tape shaping machine to yield a green sheet having thefilm thickness of 220 μm after drying. The green sheet was cut intodimensions of 2.0 cm×2.0 cm×0.022 cm and fired at 1300° C. for two hoursto yield a porous substrate made of alumina.

The porosity of the porous substrate was determined to be 40% byArchimedes' method.

The mean pore size of the porous substrate was also determined to be 0.3μm. In the present invention, the mean pore size was determined bymeasuring the longest dimension of each pore based on the scanningelectron microscopic (SEM) image of the surface of the porous substrate.The SEM image was observed at 20,000-fold magnification. All themeasured pore sizes are listed in order of size to calculate theaverage, from which the subsequent 15 larger sizes and the subsequent 15smaller sizes, i.e., 30 diameters in total, are selected in one field ofview. The selected sizes of two fields of view are then averaged toyield the average pore size. In the measurement, a dimension measuringfunction in software of SEM was used.

(2) Spin Coating and Sulfonation of Polystyrene

A polystyrene substrate (0.6 g) was dissolved in a xylene solution (10mL) to prepare a spin coating solution having a polystyreneconcentration of 0.06 g/mL. The resulting spin coat solution (0.1 mL)was dropwise applied and spin-coated on the 8YSZ porous substrate at arotation rate of 8,000 rpm. The spin coating was continued for 200seconds including the dropwise application and drying. The poroussubstrate coated with the spin coating solution was sulfonated in 95%sulfuric acid at 25° C. for four days.

(3) Preparation of Aqueous Raw Material Solution Magnesium nitratehexahydrate (Mg(NO₃)₂.6H₂O, manufactured by Kanto Chemical CO., Inc.),aluminum nitrate nonahydrate (Al(NO₃)₃.9H₂O, manufactured by KantoChemical CO., Inc.), and urea ((NH₂)₂CO, manufactured by Sigma-AldrichCorporation) were provided as raw materials. Magnesium nitratehexahydrate and aluminum nitrate nonahydrate were weighed such that acation ratio (Mg²⁺/Al³⁺) was 2 and a molar concentration of the totalmetal ions (Mg²⁺+Al³⁺) was 0.320 mol/L to be placed in a beaker.Ion-exchanged water was added thereto into a total amount of 70 mL.After stirring the solution, the urea weighed at a urea/NO₃ ⁻ molarratio of 4 was added, and further stirred to yield an aqueous rawmaterial solution.

(4) Formation of Membrane by Hydrothermal Treatment

The aqueous raw material solution prepared in Procedure (3) and thesubstrate prepared in Procedure (2) were placed in a Teflon™ hermeticcontainer (autoclave, the internal volume: 100 mL, and covered withstainless steel jacket). The substrate was horizontally fixed away fromthe bottom of Teflon™ hermetic container such that the solution was incontact with the two surfaces of the substrate. A LDH oriented membranewas then formed on the surface of the substrate by a hydrothermaltreatment at a temperature of 70° C. for 168 hours (or seven days).After a predetermined period, the substrate was removed from thehermetic container, washed with ion-exchanged water, and dried at 70° C.for ten hours to give a LDH containing functional layer partly embeddedin the porous substrate. The thickness of the functional layer was about3 μm (including the thickness of the portion embedded in the poroussubstrate).

(5) Results of Evaluation

Evaluations 1 to 3 and 5 to 8 were performed on the resultant functionallayer or composite material. The results were as follows.

-   -   Evaluation 1: The resulting XRD profile identifies the        functional layer as a LDH (hydrotalcite compound).    -   Evaluation 2: FIGS. 14A and 14B illustrates SEM images showing        the surface microstructure and the cross-sectional        microstructure of the functional layer, respectively. Similar to        the functional layers in Example 1, these figures illustrate a        functional layer composed of a membrane portion made of an LDH        membrane and a composite portion composed of the LDH and a        porous substrate located below the membrane portion.    -   Evaluation 3: The results of EDS elemental analysis indicate        that C, Mg and Al that are constituent elements of the LDH are        detected in the LDH contained in the functional layer or in both        the LDH membrane on the substrate surface and the LDH portion in        the substrate. Mg and Al are constituent elements of the basic        hydroxide layer while C corresponds to CO₃ ²⁻ that is an anion        constituting the intermediate layer of LDH.    -   Evaluation 5: FIG. 15 illustrates SEM images showing the surface        microstructure of the functional layer before immersion and        after immersion for one week in an aqueous potassium hydroxide        solution. In FIG. 15, a change in the microstructure of the        functional layer was observed even after immersion in the        aqueous potassium hydroxide solution at 30° C. lower than 70° C.        of Examples 1 to 5 for one week (that is, even under milder        alkaline condition than in Example 1). In addition, FIG. 16        illustrates the X-ray diffraction patterns of the functional        layer before immersion and after immersion for one week in the        aqueous potassium hydroxide solution. In FIG. 16, a change in        the crystalline structure was observed even after immersion in        the aqueous potassium hydroxide solution at 30° C. lower than        70° C. for one week (that is, even under milder alkaline        condition than in Examples 1 to 5). In particular, the (003)        peak of the LDH contained in the functional layer was shifted        from 2θ=11.70 before immersion to 2θ=11.44 after immersion for        one week in the aqueous potassium hydroxide solution. The shift        of the (003) peak can suggest that Al contained in the LDH is        eluted into the aqueous potassium hydroxide solution to        deteriorate the LDH. These results indicate that the functional        layer of Example 6 is inferior in alkaline resistance to the        functional layers of Example 1. In summary, the functional        layers of Example 1 in the present embodiment are superior in        alkaline resistance to the functional layer of Example 6 of the        comparative example.    -   Evaluation 6: A conductivity of the functional layer was 2.0        mS/cm.    -   Evaluation 7: The functional layer and the composite material        were confirmed to have high density with no air permeability.    -   Evaluation 8: Helium permeability through the functional layer        and the composite material was 0.0 cm/min·atm.

TABLE 1 Evaluation of alkaline resistance EDS Results Coating elementalof XRD conditions analysis (003) of after Results of SEM peak shift**alumina/ formation observation beyond titania of LDH Change in 0.25° solmembrane Immersing Change in cross- between Molar Atomic conditionssurface sectional before ratio ratio of Temp. Time micro- micro- andafter Ti/Al Ti/(Ni + Ti + Al) (° C.) (weeks) structure structureimmersion Example 1 0.26 0.20 70 3 Not found Not found Not found 70 7Found Found Found Example 2 1 0.26 70 3 Not found Not found Not found 707 Found Found Not found Example 3 1.5 0.30 70 3 Not found Not found Notfound 70 7 Not found Not found Not found Example 4 2 0.38 70 3 Not foundNot found Not found 70 7 Not found Not found Not found Example 5 5 0.6170 3 Not found Not found Not found 70 7 Not found Not found Not foundExample 6* Mg/Al-containing LDH 30 1 Found Found Found *comparativeexample **[(003) peak (2θ) before immersion] − [(003) peak (2θ) afterimmersion] > 0.25°

Example 7

A functional layer containing a Ni/Al/Ti-containing LDH and a compositematerial were prepared with a polymeric porous substrate and evaluatedby a following procedure.

(1) Preparation of Polymeric Porous Substrate

A commercially available polypropylene porous substrate having aporosity of 50%, a mean pore size of 0.1 μm and a thickness of 20 μm wascut out into a size of 2.0 cm×2.0 cm.

(2) Coating of Alumina/Titania Sol on Polymeric Porous Substrate

An amorphous alumina solution (AI-ML15, manufactured by Taki ChemicalCo., Ltd.) and a titanium oxide sol solution (M6, manufactured by TakiChemical Co., Ltd.) were mixed at Ti/Al molar ratio of 2 to yield amixed sol. The mixed sol was applied onto the substrate prepared inProcedure (1) by dip coating. In dip coating, the substrate was immersedin 100 mL of the mixed sol, pulled up vertically and dried in a dryer at90° C. for five minutes.

(3) Preparation of Aqueous Raw Material Solution

An aqueous raw material solution was prepared as Procedure (3) inExample 1.

(4) Formation of Membrane by Hydrothermal Treatment

The aqueous raw material solution and the dip-coated substrate wereplaced in a Teflon™ hermetic container (autoclave, the internal volume:100 mL, and covered with stainless steel jacket). The substrate washorizontally fixed away from the bottom of a Teflon™ hermetic containersuch that the solution was in contact with the two surfaces of thesubstrate. The LDH was then formed on the surface of the substrate andin the substrate by a hydrothermal treatment at a temperature of 120° C.for 24 hours. After a predetermined period, the substrate was removedfrom the hermetic container, washed with ion-exchanged water, and driedat 70° C. for ten hours to give a LDH containing functional layerembedded into the porous substrate.

(5) Evaluation Results

Evaluations 1 to 8 were performed on the resultant functional layer orthe composite material. The results were as follows.

-   -   Evaluation 1: The resulting XRD profile identifies the        functional layer as a LDH (hydrotalcite compound).    -   Evaluation 2: FIG. 17 illustrates SEM images showing the        cross-sectional microstructure of the functional layer or the        composite material. FIG. 17 demonstrates that the functional        layer is embedded over the entire region in the thickness        direction of the porous substrate, that is, the pores of the        porous substrate are completely filled with the LDH.    -   Evaluation 3: The results of EDS elemental analysis indicate        that C, Al, Ti and Ni that are LDH constituent elements are        detected in the LDH contained in the functional layer or in both        the LDH membrane on the substrate surface and the LDH portion in        the substrate. Al, Ti and Ni are constituent elements of the        basic hydroxide layer while C corresponds to CO₃ ²⁻ that is an        anion constituting the intermediate layer of LDH.    -   Evaluation 4: The atomic ratio Ti/(Ni+Ti+Al) of the surface of        each functional layer calculated by EDS elemental analysis was        0.38.    -   Evaluation 5: No change in the microstructure of the functional        layers in Examples 1 to 5 was observed even after immersion in        an aqueous potassium hydroxide solution at 70° C. for 3 weeks or        7 weeks.    -   Evaluation 6: The conductivity of the functional layer was 2.0        mS/cm equivalent to the above Examples 1 to 6.

Evaluation 7: The functional layer and the composite material wereconfirmed to have high density with no air permeability.

Evaluation 8: Helium permeability through the functional layer and thecomposite material was 0.0 cm/min·atm.

What is claimed is:
 1. A functional layer comprising a layered doublehydroxide, wherein the layered double hydroxide is composed of: aplurality of basic hydroxide layers comprising Ni, Al, Ti, and OHgroups; and intermediate layers composed of anions and H₂O, eachintermediate layer being interposed between two adjacent basic hydroxidelayers, wherein the basic hydroxide layers are composed of Ni, Al, Ti,and OH groups, or composed of Ni, Al, Ti, OH groups and incidentalimpurities.
 2. The functional layer according to claim 1, wherein thefunctional layer has a hydroxide ion conductivity.
 3. The functionallayer according to claim 1, wherein the functional layer has an ionicconductivity of 0.1 mS/cm or more.
 4. The functional layer according toclaim 1, wherein the layered double hydroxide undergoes no change insurface microstructure and crystalline structure when immersed in a 5mol/L aqueous potassium hydroxide solution containing zinc oxide in aconcentration of 0.4 mol/L at 70° C. for three weeks.
 5. The functionallayer according to claim 1, wherein the layered double hydroxidecomprises agglomerates of platy particles, the platy surfaces of theplaty particles being oriented perpendicular or oblique to a layersurface of the functional layer.
 6. The functional layer according toclaim 1, wherein the functional layer has a helium permeability per unitarea of 10 cm/min·atm or less.
 7. The functional layer according toclaim 1, wherein the functional layer has a thickness of 100 μm or less.8. The functional layer according to claim 1, having an atomic ratioTi/(Ni+Ti+Al) of 0.10 to 0.90 determined by an energy dispersive X-rayanalysis (EDS).
 9. The functional layer according to claim 1, furthercomprising titania.
 10. A composite material comprising: a poroussubstrate, and a functional layer according to claim 1 provided on theporous substrate and/or embedded in the porous substrate.
 11. Thecomposite material according to claim 10, wherein the porous substrateis composed of a polymeric material and the functional layer is embeddedover the entire region in the thickness direction of the poroussubstrate.
 12. The composite material according to claim 10, wherein thefunctional layer has a membrane portion provided on the poroussubstrate, the layered double hydroxide composing the membrane portionincludes agglomerates of platy particles, the platy surfaces of theplaty particles being oriented perpendicular or oblique to the surfaceof the porous substrate.
 13. The composite material according to claim10, wherein the composite material has a helium permeability per unitarea of 10 cm/min·atm or less.
 14. A battery comprising, as a separator,the functional layer according to claim
 1. 15. A functional layercomprising a layered double hydroxide, wherein the layered doublehydroxide is composed of: a plurality of basic hydroxide layerscomprising Ni, Al, Ti, and OH groups; and intermediate layers composedof anions and H₂O, each intermediate layer being interposed between twoadjacent basic hydroxide layers, wherein the functional layer has anionic conductivity of 0.1 mS/cm or more.
 16. The functional layeraccording to claim 15, wherein the functional layer has a hydroxide ionconductivity.
 17. The functional layer according to claim 15, whereinthe layered double hydroxide undergoes no change in surfacemicrostructure and crystalline structure when immersed in a 5 mol/Laqueous potassium hydroxide solution containing zinc oxide in aconcentration of 0.4 mol/L at 70° C. for three weeks.
 18. The functionallayer according to claim 15, wherein the layered double hydroxidecomprises agglomerates of platy particles, the platy surfaces of theplaty particles being oriented perpendicular or oblique to a layersurface of the functional layer.
 19. The functional layer according toclaim 15, wherein the functional layer has a helium permeability perunit area of 10 cm/min·atm or less.
 20. The functional layer accordingto claim 15, wherein the functional layer has a thickness of 100 μm orless.
 21. The functional layer according to claim 15, having an atomicratio Ti/(Ni+Ti+Al) of 0.10 to 0.90 determined by an energy dispersiveX-ray analysis (EDS).
 22. The functional layer according to claim 15,further comprising titania.
 23. A composite material comprising: aporous substrate, and a functional layer according to claim 15 providedon the porous substrate and/or embedded in the porous substrate.
 24. Thecomposite material according to claim 23, wherein the porous substrateis composed of a polymeric material and the functional layer is embeddedover the entire region in the thickness direction of the poroussubstrate.
 25. The composite material according to claim 23, wherein thefunctional layer has a membrane portion provided on the poroussubstrate, the layered double hydroxide composing the membrane portionincludes agglomerates of platy particles, the platy surfaces of theplaty particles being oriented perpendicular or oblique to the surfaceof the porous substrate.
 26. The composite material according to claim23, wherein the composite material has a helium permeability per unitarea of 10 cm/min·atm or less.
 27. A battery comprising, as a separator,the functional layer according to claim
 15. 28. A functional layercomprising a layered double hydroxide, wherein the layered doublehydroxide is composed of: a plurality of basic hydroxide layerscomprising Ni, Al, Ti, and OH groups; and intermediate layers composedof anions and H₂O, each intermediate layer being interposed between twoadjacent basic hydroxide layers, wherein the layered double hydroxidecomprises agglomerates of platy particles, the platy surfaces of theplaty particles being oriented perpendicular or oblique to a layersurface of the functional layer.
 29. The functional layer according toclaim 28, wherein the functional layer has a hydroxide ion conductivity.30. The functional layer according to claim 28, wherein the layereddouble hydroxide undergoes no change in surface microstructure andcrystalline structure when immersed in a 5 mol/L aqueous potassiumhydroxide solution containing zinc oxide in a concentration of 0.4 mol/Lat 70° C. for three weeks.
 31. The functional layer according to claim28, wherein the functional layer has a helium permeability per unit areaof 10 cm/min·atm or less.
 32. The functional layer according to claim28, wherein the functional layer has a thickness of 100 μm or less. 33.The functional layer according to claim 28, having an atomic ratioTi/(Ni+Ti+Al) of 0.10 to 0.90 determined by an energy dispersive X-rayanalysis (EDS).
 34. The functional layer according to claim 28, furthercomprising titania.
 35. A composite material comprising: a poroussubstrate, and a functional layer according to claim 28 provided on theporous substrate and/or embedded in the porous substrate.
 36. Thecomposite material according to claim 35, wherein the porous substrateis composed of a polymeric material and the functional layer is embeddedover the entire region in the thickness direction of the poroussubstrate.
 37. The composite material according to claim 35, wherein thefunctional layer has a membrane portion provided on the poroussubstrate, the layered double hydroxide composing the membrane portionincludes agglomerates of platy particles, the platy surfaces of theplaty particles being oriented perpendicular or oblique to the surfaceof the porous substrate.
 38. The composite material according to claim35, wherein the composite material has a helium permeability per unitarea of 10 cm/min·atm or less.
 39. A battery comprising, as a separator,the functional layer according to claim
 28. 40. A functional layercomprising a layered double hydroxide, wherein the layered doublehydroxide is composed of: a plurality of basic hydroxide layerscomprising Ni, Al, Ti, and OH groups; and intermediate layers composedof anions and H₂O, each intermediate layer being interposed between twoadjacent basic hydroxide layers, wherein the functional layer has ahelium permeability per unit area of 10 cm/min·atm or less.
 41. Thefunctional layer according to claim 40, wherein the functional layer hasa hydroxide ion conductivity.
 42. The functional layer according toclaim 40, wherein the layered double hydroxide undergoes no change insurface microstructure and crystalline structure when immersed in a 5mol/L aqueous potassium hydroxide solution containing zinc oxide in aconcentration of 0.4 mol/L at 70° C. for three weeks.
 43. The functionallayer according to claim 40, wherein the functional layer has athickness of 100 μm or less.
 44. The functional layer according to claim40, having an atomic ratio Ti/(Ni+Ti+Al) of 0.10 to 0.90 determined byan energy dispersive X-ray analysis (EDS).
 45. The functional layeraccording to claim 40, further comprising titania.
 46. A compositematerial comprising: a porous substrate, and a functional layeraccording to claim 40 provided on the porous substrate and/or embeddedin the porous substrate.
 47. The composite material according to claim46, wherein the porous substrate is composed of a polymeric material andthe functional layer is embedded over the entire region in the thicknessdirection of the porous substrate.
 48. The composite material accordingto claim 46, wherein the functional layer has a membrane portionprovided on the porous substrate, the layered double hydroxide composingthe membrane portion includes agglomerates of platy particles, the platysurfaces of the platy particles being oriented perpendicular or obliqueto the surface of the porous substrate.
 49. The composite materialaccording to claim 46, wherein the composite material has a heliumpermeability per unit area of 10 cm/min·atm or less.
 50. A batterycomprising, as a separator, the functional layer according to claim 40.51. A functional layer comprising a layered double hydroxide, whereinthe layered double hydroxide is composed of: a plurality of basichydroxide layers comprising Ni, Al, Ti, and OH groups; and intermediatelayers composed of anions and H₂O, each intermediate layer beinginterposed between two adjacent basic hydroxide layers, wherein thefunctional layer has an atomic ratio Ti/(Ni+Ti+Al) of 0.10 to 0.90determined by an energy dispersive X-ray analysis (EDS).
 52. Thefunctional layer according to claim 51, wherein the functional layer hasa hydroxide ion conductivity.
 53. The functional layer according toclaim 51, wherein the layered double hydroxide undergoes no change insurface microstructure and crystalline structure when immersed in a 5mol/L aqueous potassium hydroxide solution containing zinc oxide in aconcentration of 0.4 mol/L at 70° C. for three weeks.
 54. The functionallayer according to claim 51, wherein the functional layer has athickness of 100 μm or less.
 55. The functional layer according to claim51, further comprising titania.
 56. A composite material comprising: aporous substrate, and a functional layer according to claim 51 providedon the porous substrate and/or embedded in the porous substrate.
 57. Thecomposite material according to claim 56, wherein the porous substrateis composed of a polymeric material and the functional layer is embeddedover the entire region in the thickness direction of the poroussubstrate.
 58. The composite material according to claim 56, wherein thefunctional layer has a membrane portion provided on the poroussubstrate, the layered double hydroxide composing the membrane portionincludes agglomerates of platy particles, the platy surfaces of theplaty particles being oriented perpendicular or oblique to the surfaceof the porous substrate.
 59. The composite material according to claim56, wherein the composite material has a helium permeability per unitarea of 10 cm/min·atm or less.
 60. A battery comprising, as a separator,the functional layer according to claim 51.